Compact linear accelerator



Jan. 26, 1960 J. c. NYGARD COMPACT LINEAR ACCELERATOR 2 Sheets-Sheet 1Filed Oct. 28, 1954 Jan. 26, 1960 J. c. NYGARD 2,922,921

COMPACT LINEAR ACCELERATOR Filed Oct. 28, 1954 2 Sheets-Sheet 2 FIG. 3

United States Patent 2,922,921 COMPACT LINEAR ACCELERATOR John C.Nygard, Waltham, Mass., assignor to High Voltage EngineeringCorporation, Cambridge, Mass., a corporation of MassachusettsApplication October 28, 1954, Serial No. 465,327 3 Claims. (Cl.315--'5.42)

This invention relates to microwave linear accelerators for theacceleration of electrons to high energy, and in particular to amicrowave linear accelerator wherein an electron discharge device whichis used to supply highfrequency power to the acceleration tube orwaveguide of the linear accelerator is simultaneously used to injectelectrons into the waveguide, thereby eliminating the nec'essity for aseparate electron injector.

In a microwave linear accelerator electrons are injected into one end ofa Waveguide in which a traveling electromagnetic wave is produced bymeans of highfrequency power fed into the waveguide from a highfrequencyoscillator such as a magnetron or klystron. The apparatus is designed sothat the electromagnetic wave has an axial electric field component, andthe Waveguide is iris-loaded so that the phase velocity of the travelingwave is very nearly equal to the velocity of light in vacuo, or 3 10cm./sec., The electrons are injected into the, waveguide at a velocitywhich is almost equal to the velocity of light, so that the electronsstart traveling down the waveguide with almost the same velocity as thephase velocity ofthe wav Some of the electrons will find themselves inanaccelerating electric field which raises their velocity to a valuevery nearly equal to the velocity of light. These electrons can thengain only a negligible amount of additional velocity, and so they remainin the accelerating field throughout the length of the waveguide,continually gaining energy from the wave in the form'of an increase inthe electrons mass. If this accelerating field is E volts/cm. and if thewaveguide is L cm. long, the electrons in the field will issue from thewaveguide with an energy (E+ EL) electron volts, where E is the energyofthe] electronsat injection into the Waveguide. j

As stated, the traveling wave is produced by means of ahigh-frequencyoscillator, such as a magnetron or a klystron, whosehigh-frequency power output is delivered to the waveguide in such a wayas to produce the desired wave. In some cases where added power isdesired, the output from the oscillator may be amplified by one or morehigh-frequency amplifiers, such as klystrons. i

- My invention relates to microwave linear accelerators wherein thehigh-frequency power source for the- Waveguide includes "at least oneelectron discharge device which produces a'n electron beam, andmy'invention comprehends the simultaneous use of such an electrondischarge device as the electron injector for the linearaccelerator.

In the preferred embodiment of my invention, a klystron which forms apart of the high-frequency power supply is simultaneously used as theelectron injector. By using a klystron as the'dual-purpose electrondischarge device, the electron beam may be injected into the waveguidein such away that a maxi-mum number of electrons receive optimumacceleration in the waveguide. However, my invention also' comprehendsthe use of other high-frequency power units as the electron injector,such as a traveling-wave amplifier tube. High-frequency 2 power units,such as magnetrons, which do not produce an electron beam, are notsuited'to use as the dual purpose electron discharge device of myinvention.

In the drawings:

Fig. 1 is a diagrammatic view in cross-section of a microwave linearaccelerator in which, in accordance with my invention, a klystronoscillator simultaneously serves as the high-frequency power source andas the electron injector; t

'Fig. 2 is a diagram illustrating the general configuration of the linesof force of the electric field within the waveguide of the linearaccelerator of Fig. 1;

Fig. 3 is a view similar to that of Fig. 1, showing a microwave linearaccelerator in which, in accordance with my invention, a klystronamplifier simultaneously serves as a part of the high-frequency powersource and as the electron injector; and

Fig. 4 is a view similar to that of Fig. 1, showing a,

microwave linear accelerator in which a traveling-wave amplifiersimultaneously serves as a part of the highfrequency power source and asthe electron injector.

Referring to the drawings and first to Fig. 1' thereof, .therein isdiagrammatically shown a waveguide 1 of a conventional microwave linearaccelerator. High-frequency power, for the production of the'desiredtraveling wave in the waveguide 1, is fed to the waveguide 1 from amultiple-resonator klystron oscillator 2 by a suitable transmission line3. Except as noted hereinafter, the

klystron 2 may be of a conventional type. Electrons emitted from acathode 4, to which-a negative potential is applied by voltage source 5,are accelerated toward a pair of grids 6 which constitutte a part of thebounding surface of an input cavity resonator 7. The acceleratedelectrons are maintained as an electron beam 8 by means of focusingmagnets 9 provided at intervals along the length of the klystron 2.

' The resonator 7 is tuned to the desired frequency f, and is sodesigned that electromagnetic oscillations therein create ahigh-frequency electric field between the grids 6 and perpendicularthereto. A high-frequency signal,

having the frequency f, is fed into the input resonator 7 by atransmission line 10, and the resultant oscillations in the resonator 7create an electric field between the grids 6 which alternatelyaccelerates and decelerates the electrons traveling therebetween. Theinput resonator 7 thus velocity modulates the electron beam 8.

After emerging from the grids 6, the electrons travel, through' a firstdrift-space 11, in which the accelerated electrons catch up to theunaccelerated electrons preced-,

ing them, and the decelerated electrons drop back to the unacceleratedelectrons following them, so that the elec-. tron beam 8 becomesbunched. The bunched beam then travels through a second pair of grids12, which constitute a part of the bounding surface of a second cavityresonator 13. The second resonator 13 is substantially identical to theinput resonator 7, and is tuned to the same frequency f. The passage ofthe electron beam 8 in bunched form through the second resonator 13induces oscillations in the second resonator 13, and the resultanthigh-frequency electric field between the grids 12 velocity-modulatesthe electron beam 8 still more.

, After emerging from the grids 12, the electrons travel through asecond drift space 14, in which still more bunching of the electron beam8 occurs.- The highly bunched beam then travels through a third pair ofgrids. 15 which constitute a part of the bounding surface of an outputcavity resonator 16. The output resonator 16 is substantially identicalto the other two resonators 7, 13, and is tuned to the same frequency f.The passage of the highly bunched electron beam 8 through the outputresonator 16 induces oscillations in the output 3 resonator 16 which areof much greater amplitude than the oscillations in the input resonator7.

In this manner, the klystron 2 has amplified the signal which was fedinto the input resonator 7. In the apparatus of Fig. l, the klystron 2is to operate as an oscillator, and so a portion of the high frequencyoutput from th'e'out'put resonator 16 is fed back to the input resonator7 through the transmission line 10. Since a klystron is a relativelybroad-band device, a high-Q cavity resonator 17 should be included inthe feed back transmission line 10 for proper frequency control. Therest of the highfrequency output from the output resonator 16 is fed tothe waveguide 1 through the transmission line 3.

In a microwave linear accelerator, the frequency of the traveling waveis preferably as high as possible. A frequency as high as 10,000megacycles would be desirable, but the power tubes currently availablelimit the frequency attainable. At the present time, a frequency of3,000 megacycles may be taken as representative, which corresponds to awavelength of 10 cm. Maximum highfrequency power is desired, and so theklystron 2 should be designed so as to give maximum power. At thepresent time, klystrons delivering power of up to on the order of 10megawatts can be constructed. Using such a high-power klystron in theapparatus of Fig. l, the oathode 4 will deliver about 100 amps. and thevoltage source 5 will deliver 100 kv. Since klystrons are about 30percent efiicient at the present time, this input of megawatts willresult in an output of about 3 megawatts.

Owing to the high power involved, the klystron 2 is pulsed, rather thanbeing operated continuously. Hence, the voltage source 5 comprises amodulator giving an output of square-wave, 100-kv. pulses about 2microsecs. in duration, vw'th a pulse repetition rate of about 35 to 350p.p.s. The pulse length is seen to be very long compared with the periodof the high-frequency oscillations. In describing the operation ofmicrowave linear accelerators, reference is made throughout thedescription herein to conditions during the pulse. Thus, for example,the 3- megawatt output of the klystron hereinbefore mentioned refers tothe power output during the pulse and not to the average power output.

Referring again to Fig. l, the electron beam 8 cmerges from the gridsinto a third drift space 18. The passage through the output resonator 16results in additional velocity modulation of the electron beam 8, sothat it arrives at an anode 19 as a very highly bunched beam. Thefunction of the anode 19 is merely to collect the electrons in theelectron beam 8, and in a conventional klystron, the anode 19 is a solidconductive block having cooling means for dissipating the heat generatedby the bombardment thereof by the electron beam 8.

In accordance with my invention, and as shown in Fig. l, I provide anaperture 20 in the anode 19 in order to permit a portion 8' of theelectron beam 8 to travel through the aperture 20 and into the waveguide1, in which some of the electrons in the electron beam 8 becomeaccelerated to high energy by the traveling wave existing in thewaveguide 1. Since only a portion 8' of the electron beam passes throughthe aperture 20, the heat generated by the impinging of the rest of theelectrons in the beam 8 upon the anode 19 must be dissipated, and forthat purpose a coil 21 of copper tubing through which water iscirculated is wound about the anode 19.

Consider first the properties that We wish the electron beam 8 to haveas it is injected into the waveguide 1. Referring now to Fig. 2, thereinis shown the general configuration of the lines of force of the electricfield in the waveguide 1, with the arrows pointing in the conventionalsense, so that an electron will be accelerated against the arrows. Thewave, and hence the field pattern'shown in Fig. 2, is moving from leftto right with a phase velocity c=3 l0 cm./sec. In the followingdiscussion, it will be assumed that we are riding with the wave, so thatthe waveguide 1 is moving from right to left with the velocity c and thefield pattern shown in Fig. 2 is stationary.

Consider now an electron which enters the wave at the point A, where theelectric field is zero. Initially the electron is neither acceleratednor decelerated, and so if it enters the wave at the velocity 0, it willtravel the length of the waveguide 1 without gain or loss in energy. Infact, the electron enters the wave at a velocity less than c and so ittends to drop back to the point B. At B, however, the electron is in anaccelerating field, and thus is speeded up by the wave. If the initialvelocity of the electron is sufficiently. great, the accelerating fieldwill speed up the electron to very nearly the velocity c, so that uponreaching the point B it travels in phase with "the wave. Since theelectron cannot travel faster than c, it will remain at B and continueto gain energy from the wave in the form of an increase in the electronsmass.

If the initial velocity of the electrons is insufficiently great, itwill drop back towards the point C. If the electron drops back beyondthe point C without gaining enough velocity from the wave to keep inphase with the wave, it will then enter a decelerating field. Theelectron will therefore continue to lose velocity until it drops backbeyond the point D. The slower the initial velocity of the electron, thegreater the danger that it will continue to drop back without evergaining an appreciable amount of energy from the wave.

The foregoing discussion is greatly oversimplified, but it shows thatthe electrons should be injected into the waveguide 1 with as high avelocity as possible.

Referring again to Fig. 2, electrons between A and C are in anaccelerating field, while electrons between C and D are in adecelerating field. Since the wave travels with the velocity 0, allelectrons which move with respect to the wave will move from right toleft. An electron having a velocity less than the wave will require moretime to drop back from A to C than will be required for it to drop backfrom C to D, since the accelerating field impedes its dropping backwhile the decelerating field assists it. This means that the electronswill tend to cluster about the points A and D, with a relative scarcityof electrons about the point C. More precisely, since electrons at A andD are necessarily traveling more slowly than the wave, the electron beamwill become bunched slightly to the left of the points A and D.

Again, the foregoing discussion is greatly oversimplified, but it showsthat the electron beam in the waveguide 1 is a bunched beam.

It is now clear that the electron beam 8' of Fig. 1 is well-suited forinjection into the waveguide 1. Many of the electrons in the beam 8'will have energies on the order of twice the voltage of the voltagesource 5. Thus, if the voltage source 5 has a -kv. output, many of theelectrons in the beam 8' will have energies of about 200 kev., theadditional energy having been gained from the resonators 7, 13, 16.

As the electron beam 8 enters the output resonator 16, it will be highlybunched. The largest bunches will arrive, with a frequency f, so thatthey encounter a decelcrating electric field as they pass between thegrids 15. They therefore lose energy which is imparted to thehigh-frequency field in the output resonator 16. However, between thelargest bunches, there will arrive other bunches, with a frequency of for some multiple thereof, and most of these bunches will encounter anaccelerating field as they pass through the resonator 16. Some of theelectrons in these bunches will have already experienced a net energygain in traveling through the first two resonators 7, 13 and thus someof the electrons in the beam 8 as it emerges from the output resonator16 have energies as high as twice the klystron-injector energy.

Moreover, further bunching of the electron beam 8 takes place in thethird drift space 18, so that the electron beam 8 arrives at the anode19 as a series of bunches, the series being repeated with a frequency f,and some of the bunches being composed of electrons having energies oftwice the klystron-injector energy.

' Referring now to Fig. 2, it will be recalled that the major portion ofthe useful electron beam in the waveguide 1 is concentrated in bunchesjust to the left of the points A, D. These bunches are spaced awavelength apart. Now the electronbeam 8' (Fig. 1) which is injectedinto the waveguide 1 includes bunches of electrons having energies oftwice the klystron-injector energy, and also spaced a wavelength apart.By proper adjustment of a phase control device 22, which forms a part ofthe transmission line 3, these bunches may be injected into thewaveguide 1 so as to arrive in phase with those regions of the wavewhere bunching naturally occurs.

Thus high-energy, high-current portions of the electron beam 8' areinjected into that portion of the traveling wave which provides maximumacceleration. By my in vention, not only'is the necessity for a separateelectron injector eliminated, but the output of the linear acceleratoris maximized through the injection of high energy electron bunches intothe waveguide of the linear accelerator in phase with the acceleratingportions of the traveling wave.

-It will be recalled that, although some of the bunches in the electronbeam 8 are composed of high-energy electrons, the bunches which containthe highest-energy electrons are not necessarily the bunches whichcontain the highest current. As hereinbefore pointed out, the largestbunch (and hence the bunch containing the highest current) loses much ofits energy in passing through the output resonator 16. However, theelectron beam 8 has an average current of 100 amperes, so that eventhough the bunches containing the highest energy electrons contain acurrent which is low compared with 100 amperes, these bunches willnevertheless have a current well in excess of the current required to beinjected into the waveguide 1, which is about 1 ampere. In fact, thecurrent in the electron beam 8 is too large for injection into thewaveguide 1, and must be reduced by making the area of the aperture 20smaller than that of the electron beam 8, so that some of the electronsin the beam 8 are collected on the anode 19 and their energy dissipatedin the form of heat. The size of the aperture 20 is thus selected suchthat the desired beam current is transmitted therethrough. Thus, forexample, if the klystron beam 8 carries 100 amperes and has a diameterof /2-inch, an aperture 20 having a diameter of or less would admit anelectron beam 8 of about 1 ampere.

It has just been stated that no more than about 1 ampere need beinjected into the waveguide 1. The reason for this is the fact that thebeam current of a microwave linear accelerator can be increased for agiven high-frequency power input only at the expense of the beam energy.Thus, for example, a linear accelerator that can accelerate electrons toa maximum energy of about mev. with negligible beam current canaccelerate electrons to a maximum energy of only about 6 mev. if thebeam current is about ampere.

As hereinbefore stated, the klystron which is used as the electroninjector need not be the high-frequency oscillator, but may also be ahigh-frequency amplifier of the high-frequency power supply. Fig. 3shows a klystron amplifier 2' which injects electrons into the waveguidel in the same manner as the klystron oscillator 2 of Fig. 1. Thehigh-frequency output of a magnetron oscillator 23 is amplified by theklystron 2' and then fed into the waveguide 1. v

In the apparatus of Fig. 4, the high-frequency output of the magnetronoscillator 23 is amplified by a traveling wave tube 24, and then fedinto the waveguide 1. A portion 25' of the electron beam 25 of thetraveling wave tube 24 is directed into the waveguide 1 through anaperture 26 in an anode 27 so that the traveling-wave tube 24 servesboth as high-frequency amplifier and electron in-' jector.

The traveling wave in a traveling wave tube is similar to that withinthe waveguide of a microwave linear accelerator, and so the diagram ofFig. 2 also serves to illustrate the electric field in the travelingwave tube. However, in the case of the traveling wave tube, the fieldpattern shown in Fig. 2 moves from left to right with a phase velocitymuch less than the velocity of light, and

the electrons in the electron beam of the traveling wave tube move fromleft to right with a velocity greater than the phase velocity of thewave. As an electron moves from D to C, it is decelerated, and hencesome of its kinetic energy is converted into the electromagnetic energyof the wave. As the electron moves on from C to A, it is accelerated,and hence gains energy from the wave. However, the electron spends moretime in traveling from D to C than in traveling from C to A, so that thenet effect is a transfer of energy to the wave. Moreover, the electronstend to form bunches in the regions just to the right of points A and D.

The electron beam of the traveling-wave tube 24 of Fig. 4 is created inmuch the same manner as the electron beam 8 of the klystron 2 of Fig. 1.Thus, electrons emitted from a cathode 4 are given an initial energy ofabout kev. by a voltage source 5. However, unlike the klystron 2 (Fig.l), the traveling wave tube 24 (Fig. 4) tends to cause the electron beam25 to form bunches all of which lose energy to the wave. Hence, thetraveling-wave tube 24 does not inject electrons into the waveguide 1with as high an energy as does the klystron 2 (Fig. 1). However, theelectrons in the injected beam 25 have energies of about 50 kev., andthis will be adequate for proper acceleration in the waveguide 1.

Having thus described the preferred embodiment of my invention, togetherwith several illustrative alternate embodiments thereof, it is to beunderstood that although specific terms are employed, they are used in ageneric and descriptive sense and not for purposes of limitation, thescope of the invention being set forth in the following claims.

I claim:

1. An electron accelerator comprising in combination: an evacuatedenclosure adapted to sustain a high-frequency electromagnetic field ofsuch a nature that the electric field component of said electromagneticfield along a rectilinear path through said enclosure accelerateselectrons which travel along said rectilinear path in proper phaserelationship with said electromagnetic field; means for creating anelectron beam; a first space-resonant device supported in the path oftravel of said electron beam; means for generating electromagneticoscillations in said first space-resonant device which are adapted tovelocity-modulate said beam so that after leaving said firstspace-resonant device the electrons of the beam become concentrated in aperiodically recurring series of groups, each series including at leastone group of relatively high electron density; a second space-resonantdevice tuned to the frequency of recurrence of said series of groups andsupported in the path of travel of said electron beam, whereby saidgroups of relatively high electron density create electromagneticoscillations in said second space-resonant device by virtue of theirpassage through said second space-resonant device, said electromagneticoscillations therefore accelerating other groups which pass through saidsecond space-resonant device out of phase with said groups of relativelyhigh electron density; means for conveying the energy of theelectromagnetic oscillations in said second space-resonant device intosaid evacuated enclosure, wherein said energy is stored in the form ofthe electromagnetic field in said enclosure; means for directing atleast some of the accelerated groups into said enclosure along saidrectilinear path; and a phase shifter coupled between said secondspace-resonant device and said evacuated enclosure,

whereby the phase relationship between the electromagnetic field 'in'said enclosure and other electron groups than said groups of relativelyhigh electron density. may be adjusted 'so that said other electrongroups are accelerated by said electric field component.

2. An electron accelerator comprising in combination: an evacuatedenclosure adapted to sustain a high-frequency electromagnetic field ofsuch a nature that the electric field component of said electromagneticfield along a rectilinear path through said enclosure accelerateselectrons which travel along said rectilinear path in proper phaserelationship with said electromagnetic field; means for creating anelectron beam; a first space-resonant device supported in the path oftravel of said electron beam; means for generating electromagneticoscillations in said first space-resonant device which are adapted tovelocity-modulate said beam so that after leaving said firstspace-resonant device the electrons of the beam become concentrated in aperiodically recurring series of groups, each series including at leastone groupof relatively high electron density; a second space-resonantdevice tuned to the frequency of recurrence of said series of groups andsupported in the path of travel of said electron beam, whereby saidgroups of relatively high electron density create electromagneticoscillations in said second space-resonant device by virtue of theirpassage through said second space-resonant device, means for conveyingthe energy of'the electromagnetic oscillations' in said secondspace-resonant device into said evacuated enclosure, wherein said energyis stored in the form of the electromagnetic field in said enclosure;means for directing at least some of the groups of said series of groupsinto said enclosure along said rectilinear paths; and a phase shiftercoupled between said second spaceresonant device and said evacuatedenclosure, whereby the phase relationship between the electromagneticfield in said enclosure and said groups directed along said rectilinearpath may be adjusted so that said groups directed along said rectilinearpath are accelerated by said electric field component.

3. An electron accelerator in accordance with claim 2, wherein saidmeans for generating electromagnetic oscillations in said firstspace-resonant device includes said second space-resonant device.

References Cited in the file of this patent UNITED STATES PATENTS2,392,380 Varian Jan. 8, 1946 2,464,349 Samuel Mar. 15, 1949 2,524,252Brown Oct. 3, 1950 2,556,978 Pierce June 12, 1951 2,582,186 WillshawJan. 8, 1952 2,630,544 Tiley Mar. 13, 1953 2,767,259 Peter Oct. 16, 1956

